1. Field of the Invention
The present invention relates to a hydraulic braking system for use in an automotive vehicle, and more particularly to a hydraulic braking system which comprises a hydraulic booster for actuating a master cylinder in response to operation of a manually-operated member with a hydraulic power pressure supplied from a power source, and an auxiliary cylinder which is disposed between wheel brake cylinders and the master cylinder and which increases the hydraulic braking pressure supplied from the master cylinder to the wheel brake cylinders.
2. Description of the Prior Art
In conventional service braking systems for an automotive vehicle, there are provided a plurality of hydraulic circuits connecting wheel brake cylinders mounted on road wheels with a hydraulic braking pressure generator such as a master cylinder, so that when one of the hydraulic circuits is broken, braking operation is performed by the rest of the hydraulic circuits. In general, a tandem master cylinder is used in a conventional dual circuits system.
In order to reduce the force required to operate a manually-operated member, for example a depressing force applied on a brake pedal in braking operation, the hydraulic braking system is provided with a servo unit which is referred as a servo or a booster and which utilizes compressed air, intake manifold vacuum (for a vacuum booster), or hydraulic pressure (for a hydraulic booster) as a power source. The hydraulic booster is a booster which actuates the hydraulic braking pressure generator such as the master cylinder by the hydraulic power pressure supplied from the power source in response to depression of the brake pedal.
It has been proposed to employ the hydraulic booster, in the hydraulic braking system, as a dynamic hydraulic braking pressure generator in addition to the master cylinder. In other words, a hydraulic pressure generated by the hydraulic booster in response to depression of the brake pedal is applied directly to one of the hydraulic circuits. For example, as shown in Japanese Patent Laid-open Publication No. 59-227552, the hydraulic pressure generated by the hydraulic booster is applied to rear wheel brake cylinders in a front-rear dual circuits system in order to reduce the stroke of the brake pedal.
As for the above-described conventional hydraulic braking system, in the case where the hydraulic booster becomes insufficient to fulfill its boost function, or the case where the hydraulic power pressure disappears due to stoppage of the power source or other defects so that the hydraulic booster becomes incapable of obtaining any boost function, the braking force on the front road wheels is ensured by the master cylinder, but a large depressing force shall be applied on the brake pedal in order to obtain the necessary braking force. In the, Japanese Patent Laid-open Publication No. 62-149547, there is disclosed a system, in which a booster or an auxiliary cylinder is arranged in parallel with a hydraulic braking pressure generator and a control valve is provided for applying to wheel brake cylinders a hydraulic braking pressure from a master cylinder increased by the auxiliary cylinder when a hydraulic pressure generated in a hydraulic pressure chamber of a hydraulic booster or a power source is less than that in the master cylinder by a predetermined value or above, while applying the hydraulic braking pressure from the master cylinder to the wheel brake cylinders without increasing the hydraulic braking pressure in the case other than the above. Thereby, even if the boost function of the hydraulic booster cannot be obtained, the hydraulic braking pressure from the master cylinder is increased by the auxiliary cylinder to be applied to the wheel brake cylinders, so that a large depressing force is not necessary to be applied on the brake pedal. Further, in the above publication, various embodiments are disclosed for the auxiliary cylinder.
However, in the above described hydraulic braking system provided with the auxiliary cylinder, since the hydraulic braking pressure is increased in response to a pressure difference between the hydraulic pressure generated by the power source or the hydraulic booster and the hydraulic braking pressure from the master cylinder, even in the case where the hydraulic pressure generated by the power source or the hydraulic booster is sufficient, the hydraulic braking pressure from the master cylinder is increased when the brake pedal is further depressed, so that an excessive braking force is applied thereon. Namely, when the hydraulic pressure generated by the power source or the hydraulic booster exceeds the hydraulic braking pressure from the master cylinder, the auxiliary cylinder is inoperative. However, when a large depressing force is further applied on the brake pedal so that the hydraulic braking pressure, which exceeds the hydraulic pressure generated by the power source or the hydraulic booster is supplied from the master cylinder, and the pressure difference therebetween comes to be more than a predetermined value, then the auxiliary cylinder increases the hydraulic braking pressure. Thus, even in the case where a large braking force is not necessary, as in the case where a vehicle is stopped for example, an excessive braking force is applied to the wheel brake cylinders.